It is well known that the sumbersion of a diver beyond a certain depth requires an appropriate time for decompression to permit elimination from the diver's body of a definite percentage of absorbed gases, principally nitrogen. Some prior art instruments for enabling the calculation of the proper rate of ascent of a diver include a watch and a bathometer or depth gauge, along with a chart or table for facilitating the calculations. A number of these instruments embody watches having specially marked dials, but all appear to require some manual operations or calculations to monitor a safe rise rate. Other quite differently constructed devices contain some kind of gas-permeable membranes associated with bourdon type pressures gauges, these being intended for use by divers at depths beyong the shallow submergence of sport divers, and being complex in structure and difficult to calibrate.
Existing types of capillary or manometer depth gauges are useful at the shallow depths reached by amateur divers, but capillary gauge reading follow a generally hyperbolic form of curve and are regarded as unsuitable for deeper diving. On the other hand, bourdon type gauges, if reasonably accurate at depths exceeding about 90 feet (27.42m), become progressively inaccurate at lesser depths. In recognition of this situation, certain depth gauges include both capillary and bourdon elements to obtain the desired overalll accuracy.
The previously mentioned capillary gauges usually embody a manometric channel which is arcuate or circular in configuration. With such shapes, practical considerations limit the channel length, so that a small movement of a meniscus within the channel contiguous to the closed end corresponds to such a large pressure change as to preclude availability of the depth information essential for establishing the correct rate of ascent of a submerge diver.